The manufacture of a gear can be accomplished by numerous methods. For external gears, the most common processes are either generative (hobbing and generative grinding) or form cutting (profile milling and form grinding). Each type of process has constraints and advantages. Generative processes are constrained by the ratio of gear teeth to cutter teeth (commonly called “starts” or “threads”), which limits reductions in cycle time and improvements in gear profile quality for a given tool design. The cutters are very expensive and can only be justified by a large volume of parts. However in generative processes, because the tooth shape is not the simple conjugate of the cutter shape, one cutter may be used to generate a number of parts due to cutter commonality. Form milling, where the tooth shape is the negative of the gear tooth profile form is constrained to only cutting one or two gear teeth at a time, so it is sometimes favored for gears with a low number of teeth, but can only cut one gear design, that is, there is no cutter commonality. In form cutting, the feed rate can be slowed to improve gear quality.
A gear cutting machine is described in the article “Gear cutting with a rack form multi-tooth cutter” by Tony Jeffree found at www.jeffree.co.uk (2008). The gear cutter uses a zero lead hob but simply moves the workpiece across the hob and incrementally rotates one tooth after each pass. The Jeffree article fails to disclose a general purpose gear cutter and process that combines continuous axial and radial motion of the cutter to achieve a continuous generative operation.